The present invention relates to an electroluminescence light emission apparatus which utilizes an electroluminescence element (abbreviated in the following to EL element) functioning to emit light in response to applied voltage pulses.
Various types of electroluminescence light emission apparatus based on EL elements are known in the prior art, which utilize a drive circuit such as that disclosed in Japanese Patent Publication No. 52-45466. The general configuration of the drive circuit described in that disclosure is as shown in FIG. 1. First and second switch circuits 1 and 2 formed of series-connected transistor pairs Tr1, Tr2 and Tr3, Tr4 respectively, are each connected in parallel across the output voltage from a DC power source E. An EL element 3 is connected between the connecting points of transistors Tr1, Tr2 and transistors Tr3, Tr4. Numerals 4 and 5 denote respective drive circuits for applying voltage pulses at appropriate timings to the transistors pairs Tr2, Tr3 and Tr4, Tr1 respectively, to thereby control the timings of respective conduction intervals of the transistors Tr1 to Tr4. In this way the supply voltage from the power source E is supplied as successive pulses across the EL element 3, with these pulses being of successively alternating polarity. Light is thereby emitted by the EL element 3.
With this prior art apparatus of FIG. 1, AC pulse drive is applied across the terminals of the EL element 3, i.e. pulses of successively alternating polarity with the amplitude of each pulse being equal to the output voltage from the power source E. That is, each time the output signal from the drive circuit 4 sets the transistors Tr2 and Tr3 in the conducting state, the transistors Tr1 and Tr4 are at that time held in the non-conducting state by the output signal from drive circuit 5. Thus, current flows in a path from one terminal of the power source E through transistor Tr3, EL element 3, and transistor Tr2 to the other power source terminal, in the direction indicated by the arrow A. The terminal of the EL element 3 which is connected to the connecting point of transistors Tr3 and Tr4 is thereby set at a positive potential and the terminal which is connected to the connecting point of transistors Tr1 and Tr2 is set at a negative potential, so that the EL element 3 becomes charged to a voltage which is substantially equal to the voltage from power source E. Next, when the opposite condition is established by the drive circuits 4 and 5, i.e. the transistors Tr2 and Tr3 are in the non-conducting state and transistors Tr1, Tr4 in the conducting state, current flows through a circuit path extending from one terminal of the power source E through transistor Tr1, through the EL element 3 and through transistor Tr4, i.e. in the direction indicated by the arrow B. This direction of current flow results in discharging of the charge which had been built on the EL element 3. At the point in time at which this flow of discharging current begins, the voltage which is developed across the EL element 3 is equal to the sum of the voltage to which the element has been charged and the voltage of the power source E, i.e. is substantially equal to twice the power source voltage. This prior art circuit therefore is advantageous in that an EL element requiring a relatively high value of drive voltage can be driven by utilizing a relatively low power source voltage. However since pulses of successively alternating polarity are applied to the EL element, i.e. an AC pulse drive configuration is utilized, the apparatus is basically more complex and expensive to manufacture than an apparatus in which only pulses of a single polarity are applied across the EL element, i.e. in which DC pulse drive is utilized.
Furthermore, as can be readily understood from the above description, the transistors Tr1 and Tr3 are alternately subjected to a maximum voltage which is twice the power source voltage, so that these transistors must have a high value of withstanding voltage. It is only necessary for the transistors Tr2 and Tr4 to have a relatively low value of withstanding voltage. However since it is preferable that all four of these transistors have substantially identical switching characteristics, it will in practice be necessary for each of the transistors Tr1 to Tr4 to have similar withstanding voltage characteristics. Since transistors which have a high value of withstanding voltage will provide only a relatively low level of gain, it is necessary to apply high levels of base drive current to these transistors in order to attain satisfactory switching operation. Thus, it is a problem of such a prior art apparatus that the overall power consumption is relatively high.
Conversely, if it is attempted to reduce power consumption by applying low levels of drive current to these transistors, the switching characteristics will become poor, and deterioration of the brightness of light emitted by the EL element will result.
Another type of drive circuit, which differs from that described above in being a DC pulse drive circuit, is described in Japanese Patent Laid-open No. 54-102923, and is shown in FIG. 2. With this apparatus, the collector of a transistor Tr6 is connected to the base of a charging transistor Tr5, to control the operation of transistor Tr5 by ON/OFF switching of transistor Tr6, i.e. to control charging of a load capacitance Z through transistor Tr5. The circuit configuration can be summarized as follows. The transistors Tr5 and Tr6 are connected in series through a diode D, while the cathode of the diode D is connected to the base of transistor Tr5. A resistor R is connected from the emitter of Tr5 to the capacitive load Z, which can be an EL element. With this circuit, DC pulse drive is applied to the load Z, so that this circuit is preferable to that of FIG. 1 for the case in which a high value of power source voltage is readily available. Two transistors Tr5 and Tr6 are utilized, and the same considerations regarding withstanding voltage requirements which have been described for the transistor pairs Tr1, Tr4 and Tr3, Tr2 in the example of FIG. 1 are also applicable to transistors Tr5 and Tr6 in FIG. 2. Thus, the disadvantage of relatively high power consumption being required, due to the switching characteristics of these transistors, is also encountered with the circuit of FIG. 2.